The present invention relates to a printed circuit board having a termination of a T-shaped signal line.
Many electronic systems such as computer systems, control units, or control devices include a plurality of memory units such as read-write memories (RAMs) which are controlled by a plurality of integrated circuits (IC) such as microprocessors or controllers. The memory units and the integrated circuits are disposed on a printed circuit board and communicate with each other via signal lines also disposed on the printed circuit board.
Due to the increasingly high operating speeds of such integrated circuits, communications via the signal lines between these integrated circuits on the printed circuit board have increasingly higher data transfer rates.
In order to enable high-speed electric communications (i.e., signal transfer) at high frequencies and with steep edges, the signal lines on the printed circuit board must have a defined impedance to avoid signal reflections on the signal lines. Such signal reflections may be mitigated or eliminated by terminating the signal lines with the aid of terminating resistors. The signal lines may be terminated using serial termination (i.e., at the beginning of the signal line) or parallel termination (i.e., at the end of the signal line).
FIG. 1 illustrates a printed circuit board 10 having a known parallel termination of a T-shaped signal line 12 between (i) an output 14 of a driver 16 (e.g., an output driver) of a first integrated circuit 18, for example, of a control unit such as a microprocessor or controller and (ii) respective inputs 20, 22 of input drivers 24, 26 of second and third integrated circuits 28, 30, for example, of memory units such as read-write memories (RAM). In the case of RAMs, these inputs 20, 22 represent, for example, address inputs of dynamic random access memories (DRAMs) and are supplied from the output driver 16 of the integrated circuit 18.
The T-shaped signal line 12 has a first track conductor 32, which branches off into parallel second and third track conductors 34, 36 whose line ends are respectively connected to the inputs 20, 22. The first track conductor 32 has, for example, an impedance of 46Ω. The second and the third track conductors 34, 36 each have, for example, an impedance of 64.2Ω. These track conductors 32, 34, 36, however, are not limited to having the impedances in the aforesaid example.
A terminating resistor 38, 40 is connected to a center voltage UM at each respective line end of the T-shaped signal line 12. The center voltage UM (e.g., 0.9V) is generated by a fourth integrated circuit 42 from a supply voltage (e.g., 1.8V) for the second and the third integrated circuits 28, 30. The fourth integrated circuit 42 stabilizes the voltage even at high currents, and works both as a current source and as a current sink. The impedances of terminating resistors 38, 40 (e.g., 75Ω) are a function of the line impedance.
Disadvantageously, the fourth integrated circuit 42 adds a high degree of complexity to the termination in the partial circuit board of FIG. 1, which requires additional space on the printed circuit board, and increases manufacturing costs.
FIG. 2 illustrates a printed circuit board 100 having another known parallel termination, which is implemented without the fourth integrated circuit 42 from the circuit in FIG. 1 for generating termination voltage UM of 0.9V.
According to FIG. 2, the termination voltage (e.g., 0.9V) is generated by a voltage divider from a supply voltage UB (e.g., 1.8V) for each of the second and the third integrated circuits 28, 30. For terminating the line end connected to the input 20, the voltage divider includes resistors 39, 44, each of which has an impedance twice that of the terminating resistors 38, 40 in FIG. 1. A corresponding voltage divider that includes resistors 41, 46 forms the termination of the line end connected to input 22, where each of the resistors 40, 46 has twice the impedance value of the terminating resistors 38, 40 in FIG. 1. The two voltage dividers 38/44 and 40/46 are disposed between the supply voltage UB and a reference potential, i.e., ground GND of the second and the third integrated circuits 28, 30.
Although termination according to FIG. 2 does not use the fourth integrated circuit 42 from the circuit 10 illustrated in FIG. 1, it does require twice as many resistors as compared to the termination in FIG. 1. Disadvantageously, these additional resistors require additional installation space and increase the associated cost.